1. Field of the Invention
The present invention generally relates to a control system and method for a steer-by-wire system with the passive resistance torque.
2. Description of Related Art
In a steer-by-wire system, the mechanical linkage between the steering wheel and road wheels has been eliminated. The direction handling commands are communicated from the steering wheel to the road wheels using electric signals with no mechanical linkages. The road wheels are actuated using an electric motor-based actuation control system. The steering feel of a conventional mechanically linked steering system is simulated using an electric motor-based steering wheel control system. This electric motor-based steering wheel control system is referred to as the artificial steering feel control system or the steering wheel control system.
In general, the steering wheel control system is composed of an electric motor, a motor amplifier, steering wheel, gears or other coupling devices, the steering shaft and sensors. When the steering wheel control system is powered, a passive resistance torque is produced in the steering wheel. This passive resistance torque is especially high in a steering wheel system with mechanical gears as the coupling device between motor and steering wheel. When the vehicle driver holds and turns the steering wheel, the driver feels a sticky, passive resistance torque from the steering wheel device due to this high passive resistance torque. This passive resistance torque exists in many general motion control systems.
There are two main causes of passive resistance torque: motor back EMF voltage and inertia from the gear ratio. Although, mechanical friction also contributes to the passive resistance torque. The passive resistance torque produced by back EMF and inertia from the gear ratio, is in a direction opposite of the turning direction of the steering wheel.
When vehicle driver turns the steering wheel, a back EMF voltage is generated in the motor circuit. It is known that the back EMF tends to cancel out the applied voltage, so that, the actual voltage across the motor armature is the difference between the applied voltage and the back EMF. This difference causes a current in the armature to flow. The motor current caused by back EMF voltage flows in a direction opposite the motor control current. Thus, the motor torque produced by back EMF is in the opposite direction with the turning direction of the steering wheel.
The reflected inertia of the motor, through a gear ratio of N between the motor and the steering wheel, increases as a function of N squared times the motor inertia Jm. With the steering wheel inertia represented as Js, total inertia at the steering wheel due to the influence of the gear ratio N becomesJTotal=N2Jm+Js  (1)
The gear ratio N also increases N times the resistance torque TMbackEMF from the motor caused by the back EMF voltage. Resistance torque on the steering wheel is expressed as:TSbackEMF=NTMbackEMF  (2)
This torque is a sticky, passive resistance that the driver would feel while steering the vehicle. The passive resistance torque is not acceptable in commercial steer-by-wire systems that produce active steering feel.
In view of the above, it is apparent that there exists a need for a control system for a steer-by-wire system to compensate for passive resistance torque and to generate the active steering torque as well as other required steering functions.